In today's information-driven society, the importance of liquid crystal display device (LCD) technology is increasing. Cathode ray tube (CRT) devices have been widely used to date and have many advantages in terms of performance and price. However, CRTs also have disadvantages in terms of miniaturization and portability.
In order to replace the CRT, a lightweight and slim LCD that can provide high brightness, large-size, low power consumption, and low cost has been developed.
A widely-used twist nematic (TN) LCD includes a common electrode and a pixel electrode that are formed on a top substrate and a bottom substrate, respectively. An electric field is generated between the common electrode and the pixel electrode. Due to the electric field, liquid crystals injected between the top substrate and the bottom substrate are twisted and a predetermined image is displayed. The TN LCD, however, has a very narrow viewing angle.
To solve the narrow viewing angle problem, several kinds of new LCDs have been developed. Examples include an in-plane switching (IPS) LCD and an optically compensated birefringence (OCB) LCD.
In an IPS LCD, two electrodes are formed on a single substrate (bottom substrate) so as to drive liquid crystal molecules while maintaining them in a horizontal direction with respect to the substrate. By applying a predetermined voltage to the two electrodes, an electric field may be generated in a horizontal direction with respect to the substrate.
Accordingly, in such an IPS LCD, the major axis of the liquid crystal molecule is not aligned in a vertical direction with respect to the substrate. For this reason, compared with the TN LCD, the IPS LCD has an excellent viewing angle characteristic because the change in the birefringence of the liquid crystal with respect to a clockwise direction is small.
Hereinafter, a pixel structure of a related art IPS LCD will be described with reference to FIGS. 1, 2, 3A and 3B.
FIG. 1 is a plan view of a pixel structure in a related art IPS LCD.
Referring to FIG. 1, gate lines 1a and 1b for applying driving signals are positioned parallel to each other and perpendicular to data lines 5a and 5b for applying data signals, thereby defining a unit pixel region. A thin film transistor (TFT) T1 acting as a switching element is positioned near the intersection of the gate line 1a and the data line 5a. 
A common line 3 is positioned parallel to gate lines 1a and 1b. A plurality of common electrodes 3a extend from the common line 3 in a direction parallel to the data lines.
In the unit pixel region adjacent to the gate line 1a, a drain electrode of the TFT is extended and positioned in parallel to the gate line 1a. 
Also, in the unit pixel region, a pixel electrode 7 is arranged between the common electrodes 3a in a slit shape. The pixel electrode 7 electrically contacts with the drain electrode disposed in parallel to the gate line 1a. 
That is, the common electrodes 3a and the pixel electrode 7 are alternately arranged spaced apart from each other by a predetermined distance.
Also, the slit-shaped pixel electrode 7 is extended to a portion of the common line 3, thereby forming a storage capacitance between the common line 3 and the pixel electrode 7.
In such an IPS LCD, a horizontal electric field is formed between the pixel electrode 7 and the common electrodes 3a, and liquid crystal molecules are aligned according to the horizontal electric field. Accordingly, compared with the TN LCD, the IPS LCD has an improved viewing angle characteristic.
FIG. 2 is an equivalent circuit diagram of the pixel region illustrated in FIG. 1.
Referring to FIG. 2, in the unit pixel, one switching element TFT is formed near the intersection of gate lines VG(n) and VG(n-1) and data lines. A storage capacitance Cstg and a liquid crystal capacitance CLC are connected to the TFT in parallel.
The storage capacitance Cstg is formed between the pixel electrode and the common electrode, and the liquid crystal capacitance CLC is a static capacitance applied to a liquid crystal layer. Also, a parasitic capacitance Cgs is formed between a gate electrode and a drain electrode of the TFT.
In the above-described LCD, when the TFT is turned on, a pixel voltage from the data line is applied to the pixel electrode. On the contrary, when the TFT is turned off, the pixel voltage is constantly maintained due to the static capacitances CLC and Cstg until a next pixel region is turned on.
However, when the TFT changes from the turned-on state to the turned-off state, a level shift voltage ΔVP is generated, thereby decreasing the pixel voltage. The level shift voltage ΔVP generated when a white voltage is applied to the pixel region is different from the level shift voltage ΔVP generated when a black voltage is applied to the pixel region. This relationship is expressed as
      ∇          V      G        =            (                        V                      G            ,            ON                          -                  V                      G            ,            OFF                              )        ×                  C        GS                    (                              C            LC                    +                      C            STG                    +                      C            GS                          )            
In the above equation, CLC is not a constant, but a static capacitance that changes depending on the voltage applied to the liquid crystal layer. Also, CLC changes depending on the characteristics of the liquid crystal, pixel design value, and process deviation. That is, when a sufficient voltage is applied to the liquid crystal layer (white state), CLC is maximized so that the level shift voltage ΔVP is minimized.
When a voltage applied to the liquid crystal layer is minimized (black state), CLC is minimized so that the level shift voltage ΔVP is maximized.
FIGS. 3A and 3B are graphs for explaining the problem of degrading image quality due to the level shift voltage generated at the pixel region. Specifically, FIG. 3A illustrates a case in which the pixel is turned on when the white voltage is applied thereto, and FIG. 3B illustrates a case in which the pixel is turned off when the black voltage is applied thereto.
Referring to FIG. 3A, when a select signal (a white state voltage) VD applied to the pixel changes from a high state to a low state, a voltage drop is generated in the high region due to the level shift voltage ΔVP, so that a pixel voltage V(+) lower than an original voltage is formed. In the low region, a pixel voltage V(−) lower than an original voltage is formed due to the level shift voltage ΔVP, causing flicker or image-sticking.
That is, even if the white state voltage is applied, the accurate white voltage is not exhibited, resulting in failure. The reason for this is that CLC changes depending on the voltage and again influences the level shift voltage ΔVP.
In FIGS. 3A and 3B, Vcom represents the common voltage, VG,ON or VG,OFF represents a gate driving voltage, and VLC represents a pixel voltage applied to the liquid crystal layer.
Likewise, referring to FIG. 3B, when a select signal (a black state voltage) VD applied to the pixel changes from a high state to a low state, a pixel voltage V(+) lower than an original voltage is formed in the high region due to the level shift voltage ΔVP. In the low region, a pixel voltage V(−) lower than an original voltage is formed due to the level shift voltage ΔVP, causing flicker or image-sticking.